by Artisan

MY LATEST jottings from the workshop are about holes, again. I have written about locating holes and some aspects of making them in my last two jottings. This time we will finish the subject by considering how we measure their size accurately. 

For the purpose of these notes we will assume that the holes to be measured are (nominally) round.  The three parameters to be considered, apart from surface finish, are diameter, circularity and parallelism. 

How we measure these parameters will depend very largely on the length of the hole.  If it is very short (i.e. in a piece of sheet material or a shallow counterbore) the question of parallelism is unlikely to arise.  If the hole is long (e.g. an engine cylinder bore) all three parameters are likely to be of concern.

Short Holes

Let us start by considering very short holes – typically in sheet material up to about 1/8” (3mm) thick.  The range of measuring instruments available for this situation s fairly limited.  In my notes on measuring equipment in a previous article I was less than enthusiastic about slide gauges but for our present purpose they come into their own.

Figure 1 shows two vernier slide gauges with alternative internal measuring facilities.  The larger of the two instruments has a minimum internal capacity dictated by the width across the measuring surfaces when the jaws are fully closed – 0.3 inch in the case of the gauge in the picture.  This dimension must be added to the instrument reading to determine the size of the hole being measured. 

The smaller gauge has overlapping knife edge measuring surfaces which not only result in an absolute measurement without any correction to the instrument reading but enables access to very small holes.  The measuring surfaces are rather delicate however and great care must be exercised to avoid damage which would destroy the accuracy of the tool. 

Holes in thin materials can also conveniently be measured with internal micrometers of the type show in Figure 2.  These instruments also have a minimum capacity dictated by the width of the jaws when fully closed.  They usually have a measuring range of 1 inch (25mm) with the facility to extend the capacity by fitting extensions to the movable jaw as shown in the lower view in the figure.

Long Holes

So much for short holes and counterbores.  Long holes such as cylinder bores have different measuring requirements. Their diameter can, of course, be measured with the instruments described above but only the first half an inch (possibly less) of the length will be accessible.  It is not possible to check if the hole is parallel using this type of instrument. 

The choice of measuring equipment available for use with long holes depends on the diameter of the hole concerned. In many cases the tool involved will not, in itself, provide an absolute measurement but a means of transferring the measurement to another instrument – in most cases an ordinary micrometer. 

The first and most obvious tool for this purpose is an internal caliper (Figure 3). Accurate measurement using such a tool for internal measurement requires the development of a delicate “feel” for when the measuring tips are just touching the surface of the bore when the caliper is rocked slightly from side to side.  Too heavy a contact pressure will spring the arms of the caliper and result in overestimating the bore size. Obviously the micrometer used to measure the caliper setting must be adjusted until the same “feel” is obtained as when adjusting the caliper in the hole.  This method of measurement requires practice to develop the required skills but accurate results can be achieved.

For holes over about half an inch in diameter the telescopic bore gauge (Figure 4) is a better choice than simple calipers.  These gauges employ measuring contact points on the ends of spring loaded plungers which can be locked in position by means of a knurled knob on the end of the handle.  The contact pressure is controlled by the internal springs and the construction of the instrument avoids the distortion which can occur with a simple caliper. Once set the hole diameter is measured across the contact points using a micrometer.

Gauges of this type are available with a range of up to 6” (150mm).  Below about half an inch diameter small hole bore gauges are used.  A selection of these gauges is shown in Figure 3. These instruments are effectively small calipers requiring the operator to “feel” the contact pressure of the measuring surfaces as described for calipers.  The measuring surfaces are far more rigid than those of ordinary calipers however and it is relatively easy to obtain accurate results.

For holes over about 2” in diameter  internal micrometers of the type shown in Figure 5 come into their own. These employ a micrometer head similar to an ordinary outside micrometer with a measuring range of either half an inch or one inch (or the metric equivalent). 

The instrument is adjusted to the range required by fitting appropriate extension rods.  As with multi range external micrometers there is always a risk of upsetting the datum setting of the instrument when changing extension rods and it is good practice to check the calibration using an external micrometer of known accuracy.

The circularity and parallelism of long holes can be checked by multiple measurements using the tools described above but this is done more easily and quickly using a comparator type bore gauge of the type shown in Figure 6.  As with telescopic bore gauges the contact points are spring loaded onto the bore surface and held accurately across the bore diameter by guide rollers.

The instrument must be set up using an ordinary external micrometer, the dial test indicator showing deviation from the set size as the tool is moved along and rotated within the bore.  The range of the instrument is adjusted by fitting appropriate extension rods.  The range of measurement permitted by the DTI is usually about half an inch.

Most of the internal measuring instruments described above are either transfer or comparator devices and in consequence are more subject to operator error than direct measuring instruments.  These potential errors are eliminated by three point internal micrometers as show in Figure 7. 

These instruments are self centering in the hole and provide direct reading of the size on a conventional micrometer thimble.  Each instrument has a very short range however, particularly in the smaller sizes and they are very expensive. Typically a set of four instruments covering a range of 20 to 40 mm costs around £450 to £500.  It is not surprising that this type of micrometer is fairly rare in model engineers workshops!